The invention relates to the field of tread patterns for tires for vehicles of the heavy-vehicle type, and more particularly to tires fitted on the driving axles of these vehicles.
Application EP 734887 describes a tread pattern motif and in particular depicts a groove defining a leading face which is not undercut and a trailing face which is partially undercut from a certain depth onwards.
There has been proposed in European Patent 0485778 a tread pattern which reduces the abnormal sawtooth wear noted for tires fitted in particular on the driving axles of a heavy vehicle. This tread pattern comprises a plurality of blocks separated from each other by grooves of longitudinal and transverse orientation. One variant of execution set forth in this patent consists in providing for the walls of the blocks defined by the transverse grooves to form different angles (positive for the leading wall, negativexe2x80x94undercutxe2x80x94on the trailing wall) with a direction perpendicular to the running surface.
The application of such a solution (and all the more so that described in Application EP 734887) in the case of a tire of great tread thickness associated with a great depth of tread pattern motif (great depth of the cutoutsxe2x80x94both groove and incision) for fitting on driving axles of the new generations of heavy vehicles having a greater level of driving torque and braking torque is of limited interest, because owing to the specific dimensions of these blocks, when they are subjected to compressive and shearing stresses in the region of contact with the ground, they may be subject to mechanical xe2x80x9cbucklingxe2x80x9d (or alternatively rocking of the blocks), which reduces the positive effects of this type of tread pattern essentially by increasing the rate of regular wear (that is to say, the wear of the entire block).
The present invention is directed at producing a tread for a tire intended for essentially highway travel on relatively winding roads which considerably delays the appearance of irregular sawtooth wear while increasing the wear life (by increasing the thicknesses of rubber to be worn and by reducing the rate of wear). The present invention is also directed at a heavy-vehicle tire which is provided with a tread according to the invention and is intended to be fitted on driving axles.
The tread according to the invention comprises a plurality of rubber blocks defined by grooves of longitudinal and transverse general orientation, of maximum depth H, each block being provided with a contact face forming part of the running surface of the tire which is intended to come into contact with the road, a leading face and a trailing face which are formed by grooves generally of transverse orientation, said leading and trailing faces intersecting the contact face along two ridges constituting the leading and trailing edges respectively of said block.
The leading face of the blocks is inclined so as to form an angle xcex1 greater than 90xc2x0 with the contact face; in the present description, the angles are, by definition, measured within the elastomeric material constituting the tread pattern element, as can be seen in FIG. 2 (according to this convention, a lateral face is said to be undercut once the angle of this face is less than 90xc2x0).
The trailing face is composed, in the direction of the height of the block, of two successive parts which divide said trailing face into a lower part and an upper part, the upper part opening onto the running surface of the tread when new.
In order to adjust the compressive strength and of shear of the motif which is subjected to compressive and shearing stresses which are exerted on its contact face during travel of a tire fitted with a tread according to the invention, the latter is characterized in that, when new, the upper part of the trailing face of at least one tread pattern block is undercut and forms an angle xcex2 of less than 90xc2x0 with the contact face of said block, whereas the lower part of said face, which is oriented so as to form an angle xcex4 greater than 90xc2x0 with the same contact face, is provided with at least one recess which opens onto said lower part. Furthermore, each recess extends within the block at least as far as a plane perpendicular to the contact face and passing through that point of the lower part of the trailing face which is closest to the contact face.
Preferably, the recess or recesses do not extend within each block beyond a limit taken substantially as the surface tangentially extending the upper part of the trailing face radially towards the inside of the block, because when the recesses extend too far into a block, the equilibrium of strengths is no longer satisfied and the beneficial effects of the advocated arrangements are substantially reduced.
The tread pattern according to the invention makes it possible to combine both a reduction in compressive strength within the thickness of the tread pattern blocks in the vicinity of their trailing edge with the maintaining of a high level of shearing strength in a direction parallel to the running surface.
Preferably, the upper limit of the lower part of the trailing face of a block of the tread pattern according to the invention is located at a height H, measured relative to the running surface when new, and represents at least 20% and at most 80% of the total height P of said block. This upper limit is defined as being the parting line between the upper part and the lower part of the trailing face. If the height H is less than 20% of the total height P, the compressive strength of the edge of the block is too high, which adversely affects the wear performance; if this height H is greater than 80% of the total height P, the trailing edge of the bloc becomes too flexible under compression, which also adversely affects the wear performance.
It would appear that satisfactory results are obtained once the angles xcex2 and xcex4 satisfy the following relationship:             1              tan        ⁢                  xe2x80x83                ⁢        δ               greater than                   2        ⁢        H                                          (                          H              -              P                        )                    ·          tan                ⁢                  xe2x80x83                ⁢        β                        with      ⁢              :            ⁢              xe2x80x83            ⁢      δ         greater than           90      ⁢      xc2x0      ⁢              xe2x80x83            ⁢      et      ⁢              xe2x80x83            ⁢      β         less than           90      ⁢      xc2x0      
This relationship makes it possible, once an angle xcex2 has been selected for a pair of values of H and P, to find the optimum values of the angle xcex4 which ensure optimum wear performance.
One embodiment consists in providing the lower part of the trailing face with a plurality of recesses which are regularly spaced apart in the transverse direction, said recesses extending over the entire height of the lower part of the trailing face.
In order to obtain a particularly noticeable effect on the wear performance, the groove ratio (equal to the ratio between the total area of all the recesses on the lower part of the trailing face and the area of said part) is between 30% and 70%. When the value of this groove ratio is greater than 70%, the resulting softening is too great, whereas when this value is less than 30%, the lower part of the block is too rigid.
Preferably, this groove ratio is equal to 50% of the area of the lower part of the trailing face.
Preferably, the greater the total surface area of the lower part of the bearing face compared with the surface of the upper part, the greater is the selected value of this groove ratio within the above limits.